This invention relates to compositions for the photogeneration of acid. This invention also relates to processes for the photogeneration of acid.
Many processes are known using a medium which, when irradiated with electromagnetic radiation, generates an acid This acid is then used to cause a change in the properties of the medium, so that exposed and unexposed portions of the medium differ in their properties. For example, many photoresist compositions arc of this type; the acid produced upon exposure to (typically) ultraviolet radiation changes the solubility of the photoresist composition in the solution used to develop the photoresist In most conventional acid-generating photoresist processes, the sensitivity of the medium an the exposing radiation is not of major concern. Exposure is normally effected using powerful ultraviolet sources. In addition, long exposures times can usually be tolerated.
Today, many imaging processes are being developed using near Infrared radiation from semiconductor diode lasers. Semiconductor diode lasers have the advantage of being much less expensive than ultraviolet lasers. They are also well adapted for the production of high resolution images and for digital imaging processes (i.e., for producing hard copies of images stored on computers in digital farm). The cost per unit Intensity is less for an infrared producing high-resolution addressable source than for a comparable ultraviolet radiation producing source. The imaging speed of such infrared radiation using processes is presently limited by the sensitivity of the medium, and accordingly, there is a need to develop infrared sensitive imaging media with improved sensitivity.
Oftentimes, the sensitivity of photosensitive compositions can be increased if the photosensitive molecule catalyzes a secondary reaction which is not radiation-dependent, and if the photosensitive molecule also effects conversion of several molecules for each quantum of electromagnetic energy absorbed. For example, photoresist systems are (mown in which the primary photochemical reaction produces an acid, and this acid is employed to eliminate acid-labile groups in a secondary, radiation-independent reaction. See, for example, U.S. Pat. Nos. 3,923,514 and 3,915,706. Also, U.S. Pat. No. 5,094,371 discloses a radiation-sensitive mixture which contains a water-insoluble binder comprising a mixture of phenolic and novolak polymers soluble or dispersible in aqueous alkali, and an organic compound whose solubility to alkaline developer is increased by acid, and which also contains at least one acid-cleavable group, and in addition a further group which produces a strong acid upon exposure to radiation. A secondary acid generator (when used) xe2x80x9campiffiesxe2x80x9d the acid produced by an iodonium ask or other super-acid precursor, resulting in several molecules of acid being produced for each molecule of superacid originally produced by decomposition of the iodonium still. However, despite the increase in sensitivity achieved by such acid amplification, the contrast, and hence the quality of the resultant image is still governed by the additional photochemical acid generation step. Accordingly, it is desirable to secure as high a quantum yield as possible during the photochemical acid generation step.
U.S. Pat. No. 5,286,604 discloses the use of a squarylium dye as a near infrared (NIR) light-to-heat converter for the thermal cleavage of tetrahydropyran groups from derivatized polyacrylate and methacrylate polymers for application in color proofing materials. However, the sensitivity of this system is quite low, i.e., 300-600 mJ/cm2. There is no disclosure of a squarylium dye used as a spectral sensitize for latent Bronsted acid generators.
U.S. Pat No. 5,225,316 discloses the use of various classes of dyes including, but not limited to, aryl nitrones, xanthenes, anthraquinones, substituted-diaryl and triaryl methanes, methines, merocyanines, and polymethines, thiazoles, substituted- and unsubstituted-polycyclic aromatic hydrocarbons, and pyrylium dyes in combination with iodonium salts for the photochemical imagewise generation of acid to cleave tetrahydropyran groups from derivatized polyacrylate and methacrylate polymers for application in no-process printing plates. However no mention was made specifically of squarylium dyes.
European Patent Publ. No. 568,993 discloses combinations of squarylium dyes and latent Bronsted acid generators (iodonium salts, trichloromethyl-substituted triazines, etc.) for the generation of acid by exposure to visible and NIR light The acid that is generated catalyzes various imaging mechanisms including thermal crosslinking and thermal deprotection of hydrolyzable groups from polymers. Several examples describe the thermal crosslinking of phenolic resins and melamine formaldehyde resins catalyzed by acid photogenerated by combinations of squarylium dyes and tris(trichloromethyl-s-triazine). None of the squarylium dyes disclosed contain a 2,3-dihydroperimidine terminal group.
U.S. Pat. No. 5,340,699 discloses the use of NIR squarylium dyes in combination with latent Bronsted acid generators such as diphenyliodonium salts or trichloromethyl-containing molecules to generate a strong Brousted acid which is used to catalyze the thermal crosslinking of a combination of novolak resin and resole resin.
U.S. Pat. No. 5,401,607 discloses an acid-generating medium comprising an iodonium salt and a squarylium dye in which the squarylium dye absorbs in the range of 700-1200 nm. The squarylium dye preferably has an oxidation potential in methylene chloride of not greater than 500 mV relative to the saturated calomel electrode (SCE). This patent teaches that dyes having oxidation potentials greater than about 500 mV were found not an be good acid generators.
U.S. Pat No. 4,554,238 discloses the use of sensitizing dyes in the range 300-900 nm as electron donor sensitizers of nitrobenzyl-blocked surfactants an release the Bronsted acid form of the surfactant The patent states that spectral sensitizing compounds suitable for the invention include those disclosed in the art as being suitable for the spectral sensitization of photolyzable organic halogen compounds (including trichloromethyl-substituted triazines), and sulfonium and iodonium salts. NIR squarylium dyes are disclosed, but there is no teaching that squarylium dyes can sensitize latent Brousted acid generators.
K. A. Bello, S. N. Corns and J. Griffiths, J. Chem. Soc, Chem. Commun., 452-454,1993 describes the condensation of 2,3-dihydroperimidines with squaric acid to give squarylium dyes having absorption maxima near 800 nm.
In one embodiment, the present invention provides an acid-generating medium comprising:
(a) a photochemical acid progenitor; and
(b) a squarylium dye having a nucleus of the general formula: 
wherein:
Rxe2x80x2 to R4 are independently selected from hydrogen, alkyl, cycloalkyl, aralkyl, carboalkoxyalkyl and carboaryloxyalkyl groups;
X represents  less than CR5R6,  less than POR7, or  less than BOR7 
wherein:
R5 and R6 are independently selected from hydrogen, alkyl, cycloalkyl, aryl, and aralkyl groups;
or R1 and R5, and/or R2 and R6, and/or R3 and R5, and/or R4 and R6, and/or R5 and R5 represent the necessary atoms to complete a 5-, 6- or 7-membered ring; and
R7 represents an alkyl group.
It will be readily appreciated that the dyes of formula (I) may be represented by a number of different resonance structures, reflecting the many different ways in which the delocalized xcfx80-electron system may be visualized and notated In formula (I) and elsewhere in this specification, the moiety: 
represents the aromatic dication derived from cyclobutadiene. This particular notation is chosen for convenience, and allows both the end groups and the central portion of the dye molecule to be depicted in full aromatized form. It must be emphasized, however, that formula (I) is to be interpreted as including all the possible resonance forms, such as: 
and the like.
In another embodiment, this invention provides an acid generating medium comprising:
(a) a photochemical acid progenitor selected from the group consisting of diaryliodonium salts, aryldiazonium salts, and 1.3.5-tris(trichloromethyl)-s-triazines; and
(b) a squarylium dye having an oxidation potential in dichloromethane greater than or equal to 0.5 V and less than or equal to 0.8 V relative to a standard calomel electrode.
This invention also provides a process for generating acid, comprising the steps of:
(a) providing a mixture of a photochemical acid progenitor and a squarylium dye of the formula (I) disclosed earlier herein; and
(b) irradiating the mixture with radiation from a light source, preferably a laser emitting in the near infrared region (700 to 1200 nm) of the spectrum.
It is generally accepted in the field of the present invention to allow substantial substitution on the core dye structure of the present invention. Some types of substitution, especially that which improves solubility in a selected solvent, is particularly desirable. Where the term xe2x80x9cgroupxe2x80x9d or xe2x80x9ccentral nucleusxe2x80x9d is used in describing an aspect of the present invention, that term implies that any type of substitution is acceptable, as long as the basic structure is maintained. For example, xe2x80x9calkyl groupxe2x80x9d would include not only standard hydrocarbon alkyls such as methyl, ethyl, cyclohexyl, isooctyl, undecyl, etc., but would also include substituted-alkyl such as hydroxymethyl, omega-cyanopropyl, 1.2.3-trichlorohexyl, 1-carboxy-isooctyl, phenyldecyl, and the like. The term xe2x80x9calkylxe2x80x9d or xe2x80x9calkyl moietyxe2x80x9d indicates that there s no substitution on that defined component.
Other aspects, advantages, and benefits of the present invention are apparent from the detailed description, examples, and claims.